The present invention relates to polyester sheet and film production processes. More specifically, it relates to a polyester sheet production process which can suppress the adhesion of a low-molecular weight sublimate to the inside of a micro-crack when a polyester sheet is extrusion molded using a cooling roll having micro-cracks formed on the surface and which is capable of producing a high-quality sheet having excellent smoothness stably at a high speed for a long time and to a process for producing a biaxially oriented polyester film from the obtained sheet.
As means of casting a polymer sheet, there has been known a method in which a sheet product of a molten polymer extruded from an orifice-form nozzle is closely adhered to the surface of a cooling roll to be solidified by an electrostatic adhesion method or gas pressure method. A cooling roll having a smooth surface (mirror finished surface) is generally used as the cooling roll in this method, and air caught in a gap between the cooling roll surface and the sheet product must be removed when the sheet product of the molten polymer is closely adhered to the surface of the cooling roll.
The removal of the caught air becomes more difficult as the casting speed increases, thereby causing various problems. For instance, since the caught air is existent in the form of a bubble in the electrostatic adhesion method, it causes a sheet surface defect, resulting in deterioration in the smoothness of the sheet. In the gas pressure method, the caught air causes insufficient heat transmission between the sheet and the cooling roll with the result that the sheet is not cooled enough. When the sheet is not cooled enough, the sublimation of a low-molecular weight compound contained in the molten polyester continues whereby the accumulation of the low-molecular weight compound on the surface of the cooling roll becomes marked and the compound is transferred to the surface of the sheet, thereby causing an orange skin-like defect.
When the amount of the low-molecular weight compound accumulated on the surface of the cooling roll increases, insufficient heat transmission becomes more marked, thereby making high-speed casting impossible.
This problem can be improved by exchanging the cooling roll proposed by JP-A 62-196118 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) for a cooling roll having a micro-crack formed surface (micro-cracked surface). That is, since the caught air is scattered through the grooves of the micro-cracks in the electrostatic adhesion method, a bubble-form defect can be improved and the casting speed can be greatly increased. Since the caught air is also scattered through the grooves of the micro-cracks in the gas pressure method, a reduction in the heat transmission speed between the sheet and the cooling roll can be prevented and the occurrence of an orange skin-like defect can also be avoided.
However, a process for extrusion molding a polyester sheet using a cooling roll having a micro-cracked surface involves a new problem that the ventilation resistance of the micro-crack increases along the passage of time, thereby reducing the function of scattering the caught air through the grooves of the micro-crack in a short period of time. The cause of this is that a low-molecular weight compound sublimated from a sheet product is accumulated in the inside of the groove of the micro-crack, thereby clogging the groove. Therefore, the low-molecular weight compound must be removed frequently from the groove, which poses a production problem.
To remove the deposit accumulated on the surface of the cooling roll, there are known (1) a method in which a non-contact portion between the surface of a cooling roll and a polyester sheet is always cleaned by water or a solvent, and the water or solvent is dried and sucked to be removed as disclosed by JP-B 47-3917 and JP-B 48-4465 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), (2) a method in which the surface of a cooling roll is subjected to a corona treatment as disclosed by JP-A 57-51426 and (3) a method in which a deposit is decomposed and removed by irradiating the surface of a cooling roll with ultraviolet radiation as disclosed by JP-B 3-65775.
However, the above method (1) is effective for a cooling roll having a mirror finished surface but cannot be applied to a cooling roll having a micro-cracked surface because it is difficult to wash the inside of the groove and remove a liquid in the inside of the groove. The methods (2) and (3) are also effective for a cooling roll having a mirror finished surface but cannot be applied to a cooling roll having a micro-cracked surface because they have a poor effect of decomposing and removing the deposit in the inside of the groove.
In order to wash the inside of the groove of each micro-crack of the cooling roll, JP-A 10-217307 discloses a method in which the cooling roll is immersed in a chemical bath. However, this method involves problems to be solved for practical application, such as the interruption of sheet production and the complicated operation of immersing the cooling roll in a chemical bath.
Other solutions to the above problems include a method in which the width of groove of the micro-crack is increased. However, when the width of the groove is increased, new problems are expected to arise that the transfer of the micro-crack to the sheet may cause an orange skin-like defect and it is extremely difficult to form micro-cracks having a larger width uniformly on the surface of the cooling roll.
It is an object of the present invention to provide a polyester sheet production process which can reduce the frequency of interrupting sheet production by cooling a polyester sheet while the increase speed of ventilation resistance caused by the clogging of the groove of each micro-crack is controlled to a low level and which eliminates the need for bulky equipment.
It is another object of the present invention to provide a polyester sheet production process which can provide a high-quality film without transfer by using a cooling roll having the grooves of micro-cracks.
It is still another object of the present invention to provide a process for producing a biaxially oriented polyester film from a polyester sheet produced by the above process of the present invention.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a process for producing a polyester sheet by dropping a molten polyester sheet extruded from an orifice-form nozzle on a cooling roll having the grooves of a large number of micro-cracks formed on the surface, closely adhering it to the cooling roll and solidifying it on the cooling roll, wherein
the surface temperature (T, xc2x0 C.) of the molten polyester sheet 10 mm below the orifice-form nozzle is maintained at a temperature which satisfies the following expression (1):
(Tc+20)xc2x0 C.xe2x89xa6Txe2x89xa6(Tm+40)xc2x0 C.xe2x80x83xe2x80x83(1) 
wherein Tc and Tm are the falling temperature crystallization temperature (xc2x0 C.) and melting point (xc2x0 C.) of the polyester, respectively and T is as defined hereinabove, and the surface temperature of the cooling roll when it contacts the molten polyester sheet is controlled to a range of 5 to (Tgxe2x88x9220)xc2x0 C. (Tg is the glass transition temperature of the polyester) to continuously form the polyester sheet while preventing the adhesion of a sublimate from the molten polyester to the inside of the groove of each micro-crack of the cooling roll.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by a process for producing a biaxially oriented polyester film, comprising the step of biaxially orienting the polyester sheet obtained by the above process of the preset invention in a longitudinal direction and a transverse direction.